Overall weight, fuel capacity, fuel economy, lubrication oil capacity and lubrication oil economy are all important factors for achieving range (distance) and duration (flight time) in the air for a UAV. Range and duration are related in that the longer a UAV can remain in the air the further range it may have. Duration also means that a UAV can remain over a location as long as possible, which need not be at the longest range from its take off and landing point.
Oil consumption is very important for UAV applications as it is directly linked to the vehicle's available range and duration. Engines with low oil consumption have the advantage of greater range and duration for a given initial mass of oil. It is not preferable to simply increase the amount of oil because this adds to the weight of the UAV. Improved oil economy is preferred for the reasons given above.
Two stroke engines often combust a pre-mix of oil and fuel. The pre-mix comprises a proportion of oil to maintain lubrication in the cylinder, and the fuel for power. However, it is understood that pre-mixed oil and fuel does not work for direct injection engines. The injection system cannot efficiently inject the lubrication oil and fuel pre-mix to achieve an acceptable combustion and fuel economy as well as provide lubrication to the engine components.
Other strategies for lubricating the two stroke UAV engine have been devised. A mechanical oil pump can be driven from the crankshaft. However, oil delivery is tied to engine speed. The faster the engine speed, the faster the pump operates, though giving the same volume of oil per pump stroke. Thus, the mechanically driven pump may under deliver or over deliver lubrication oil for a given engine speed. Under delivery of oil results in insufficient engine lubrication which causes poor engine reliability and durability. Over delivery causes poor oil economy because more oil than necessary is delivered for lubrication purposes, and the UAV engine will have poor oil consumption and hence reduced UAV range or duration.
The actual lubricant requirements of a UAV engine are also a function of the altitude at which the UAV operates, the engine temperature, and the engine speed or load demand (represented by position of the throttle, or a direct command from the operator). Very high altitude capability is a key differentiator between UAV engines and other engines with oil pumps.
Since UAVs are often operated at very high altitudes or in regions where wind chill creates extremely low temperatures, and also in regions where temperatures can vary greatly in winter even at ground level (which itself may be at a relatively high altitude), the actual lubricant requirements of the engine can be significantly affected by these factors and therefore need to be taken into account.
Conventional UAV lubrication systems using mechanical oil pumps or 2 stroke pre-mix, due to the linear relationship between the engine speed and the amount of lubricant being pumped, cannot take all of these into account.
For a two-stroke UAV engine, the actual lubricant requirement depends, at least in part, on the power output of the engine, not engine speed. The higher the power output, the more lubricant is required. There are instances during the operation of the two-stroke engine where the engine speed is high, but where the power output of the engine is low. In such instances, a mechanical oil pump driven by the engine would provide a lot of lubricant even though the actual requirements are low.
There are other instances where the actual lubricant requirements are lower than what would be provided by a mechanical oil pump driven by the engine. For example, at start-up, all of the lubricant that was present in the engine when it was stopped has accumulated at the bottom of the crankcase. The accumulated lubricant would be sufficient to lubricate the engine for the first few minutes of operation; however a mechanical oil pump, due to its connection to the engine, would add lubricant regardless.
Therefore, in the case of a two-stoke engine, using a mechanical oil pump results in more lubricant being consumed by the engine than is actually required under such operating conditions. This also results in a level of exhaust emissions that is higher than a level of exhaust emissions that would result from supplying the engine with its actual lubricant requirements since more lubricant gets combusted than is necessary.
Low viscosity of oil for lubrication is another factor relevant to certain engines which are required to operate in environments with cold ambient conditions as this can lead to problems with delivery of the oil to the engine at certain times. Preheating of lubrication oil is one know method for helping to alleviate this problem and one known example of preheating lubrication oil is disclosed in published patent application US 20070246302. An electric motor is used to preheat the oil. The rotor of the electric motor is either locked to cause heating or the rotor is spun without the pump operating in order to generate heat. In the former case, an additional locking mechanism is required to ensure that the rotor does not spin and otherwise cause oil to be pumped. In the latter case, the rotor and pump mechanism require an additional disengagement mechanism to prevent the spinning rotor from operating the pump and pumping oil. In either scenario, there is an inherent real risk of the additional (locking or disengagement) mechanisms failing and preventing the pump from operating as a lubrication pump or not being able to act as a heating source. The present invention avoids such problems.
Furthermore, US 20070246302 is generally directed to overcoming cold, viscous oil in a reservoir during cold start-up of gas turbine aircraft engines. Once started and warm, the pump motor form of heating is not required.
Still further, gas turbine engines of the type discussed in US 20070246302 typically have considerable waste heat associated with their normal running operation. Hence, there is typically sufficient heat during normal running to maintain the temperature of the lubricating oil to prevent the oil thickening below accepted limits.
Thus, US 20070246302 is directed to heating the oil during turbine start-up but not once the engine is running. The present invention is directed to heating oil for lubricating two stroke and four stroke fuel injected UAV internal combustion (piston) engines. Such engines operate in extreme conditions, running at much lower revolutions than gas turbine engines and therefore not generating such high levels of heat once running. An electric solenoid actuated lubrication oil pump of the present invention can operate according to a control strategy to provide additional heat to the lubrication oil at start-up and during engine running. This is particularly beneficial for high altitude or long duration running in cold climates.
With the aforementioned in mind, it would also be beneficial to provide improved control of lubrication oil supply for increased range and duration of UAVs, particularly 2 stroke UAV engines utilising direct injection and/or for an air compressor of a dual fluid (air assisted) direct injection engine.
More particularly, it is desirable to provide a lubrication system that provides an amount of lubricant that is at or near the actual lubricant requirements of the UAV engine for given conditions.